1. Field of the Invention
This invention relates to a signal processing apparatus used for digital image information transmitting systems for transmitting various image information such as videotex or teletext information by utilizing telephone circuits or radio channels and, more particularly, to an apparatus for encoding an image signal, wherein one image frame is handled as an aggregate of geometric areas and image information of each geometric area is converted into geometric command data.
2. Description of the Prior Art
With the recent development of the information communication society, digital image information transmitting systems for transmitting videotex and teletext information have been developed and used in practice as new media of transmission of various image information in various countries. For example, a system called PRESTEL has already been in practical use in England. In Japan, a CAPTAIN (Character and Pattern Telephone Acess Information Network) system has been developed. Further, a TELETEL system in France and a NAPLPS (North American Presentation Leved Protocol Syntax) system in Canada and U.S.A. are going to be in practical use.
In the NAPLPS system, one image frame is handled as an aggregate of geometric areas and image ihformation is transmitted in the form of geometric command data based on PDI (Picture Description Instruction) codes. The high efficiency of the system is highly evaluated. In the PDI codes, the command data as "POINT", "LINE", "ARC", "RECTANGLE" and "POLYGON" are defined as geometric drawing commands. The command data "POINT" instructs setting a point of the start of a drawing or plotting a point Po at a given co-ordinate position on a display screen as shown in FIG. 1A. The command data "LINE" instructs drawing a line segment connecting two points P.sub.1 and P.sub.2 as shown in FIG. 1B. The command data "ARC" instructs drawing an arc on the basis of the co-ordinates of three points P.sub.1, P.sub.2 and P.sub.3 as shown in FIG. 1C, and also drawing a chord connecting the two points P.sub.1 and P.sub.3 as shown by a phantom line and coloring the area enclosed by the arc and chord. The command data "RECTANGLE" instructs drawing a rectangle having a pair of diagonally located vertexes at points P.sub.1 and P.sub.2 as shown in FIG. 1D and coloring the area enclosed by the rectangle. The command data "POLYGON" instructs drawing a polygon having vertexes P.sub.1, P.sub.2, . . . , Pn as shown in FIG. 1E and coloring the area enclosed by the polygon.
With the above digital image information transmitting system utilizing geometric command data, it is possible to greatly reduce the information amount of image information which is actually transmitted, thus permitting high efficiency information transmission. However, the operation of forming the geometric command data for image information, i.e., one frame of image, to be actually transmitted requires a great deal of time and labor.
For example, the conversion of a video signal obtained by picking up an image to be transmitted into the geometric command data may be carried out by an operator by scrupulously providing contour information and color hue and gradation information using tablets while watching a monitor television receiver so that the input information is progressively converted to the geometric command data. In such a method, however, it is difficult to properly and accurately express the information of the original image, and also a great deal of time and labor is necessary for providing various kinds of information.